In high speed optoelectronic applications, such as optical communications, optoelectronic devices (e.g., PIN photodiodes, semiconductor lasers, and integrated optical modulators) typically have low input resistance (e.g., on the order of 5-20 ohms). However, these devices typically are driven by 50 ohm microwave sources. The resulting impedance mismatch between the source and the optoelectronic device limits the system bandwidth and reduces the power coupling efficiency between the source and the optoelectronic device, making it difficult to generate the short optical pulses (e.g., pulses on the order of 10xe2x88x9212 to 10xe2x88x9215 seconds in width) that typically are needed for high speed operation.
The impedance mismatch problem in microwave circuits typically is solved by incorporating an impedance matching transformer between the source and the optoelectronic device. Among common microwave impedance matching transformers are single- and double-tuned stubs, lumped element transformers, and tapered line transformers. Planar tapered line transformers typically are characterized by a relatively large bandwidth, whereas other kinds of transformers typically are tuned to a specific frequency. In a planar tapered line transformer, an impedance match typically is achieved by varying the width of the signal carrying conductor (i.e., the electrode) relative to the ground plane, while maintaining a constant height along the length of the transformer. Typically, the width of the electrode is varied to continuously adjust the characteristic impedance of the transformer as a function of position along the length of the transformer.
In some optoelectronic systems, in addition to the impedance mismatch between the source and the optoelectronic device, system bandwidth often is limited by parasitic capacitance that is introduced by the bonding pad and by parasitic inductance that is introduced by the bonding wire that is attached to the bonding pad. A relatively large bonding pad typically is used to achieve good adhesion of the pad to the supporting substrate and the bonding wire. For example, a typical bonding pad has an area that is on the order of 50 micrometers by 50 micrometers. Such a large bonding pad size, however, typically results in a relatively large capacitance that may limit the bandwidth of the system and may increase the impedance mismatch to the source.
A typical approach to reducing the impact of the bonding pad and bonding wire on system performance is to reduce the parasitic capacitance of the bonding pad (e.g., by using a thick layer of a low dielectric constant material, such as polyamide) and to reduce the length of the bonding wire to reduce the parasitic inductance.
In another approach, the bonding wires are constructed to operate as transmission lines and the bonding pad is impedance-matched to the source. In this approach, the size of the bonding pad structures and the length of the bonding wires are not limiting factors because the parasitic capacitance and parasitic inductance do not impact system performance. The resulting structures in this approach, however, typically are much larger than the optoelectronic device, making it difficult to construct an impedance matching circuit between the is bonding pad and the optoelectronic device.
In one aspect, the invention features a device interconnect system that includes a bonding pad portion and a transmission line portion. The bonding pad portion is disposed on a device substrate and is constructed and arranged for electrical connection to a bond wire. The transmission line portion is disposed on the device substrate and is constructed and arranged to electrically couple the bonding pad portion to a device formed on the device substrate. The transmission line portion has a width dimension that is substantially parallel to the device substrate and a height dimension that is substantially perpendicular to the device substrate. The width dimension and the height dimension of the transmission line portion both vary from the bonding pad portion to the device.
In another aspect, the invention features a method of making the above-described device interconnect system.
Among the advantages of the invention are the following.
The invention enables the characteristic impedance to change continuously from the bonding pad portion to the device, resulting in high coupling efficiency and low insertion loss. The invention also enables reflections at the bonding pad portion and the device to be reduced because the transmission line portion may be optimized for both structures, which may have different heights. In addition, the invention enables the overall length of the transmission line portion to be reduced for a given bandwidth because both the height and width dimensions may be varied to achieve a prescribed impedance variation from the bonding pad portion to the device.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.